For now, one version isn't better than the other in every department; you'll have to pick and choose the factors that are most beneficial to you. If you want the full gamut of imaging options, however, stick with the Android model for now. HTC's signature Zoe feature, which takes a few seconds of video and burst-mode shots and converts it into a Harry Potter-esque moving picture, isn't available; the Zoe app, which is a social network that lets you show off your Video Highlights to friends and family, is also restricted to Android 4.4. Lastly, the Windows edition lacks a few of the M8's Duo Camera editing tools. It comes with UFocus (which lets you change the area of focus), Foregrounder (adds funky visual effects to objects in the background), Dimension Plus (gives the picture a faux-3D feel), rotate/crop tools, most filters and a few frames. Copy/paste, stickers, draw/flip/straighten tools and seasons (a pointless feature that shows leaves, snowflakes, etc. falling down in front of your picture) didn't make the cut, but I rarely use those features on the Android version anyway.
Performance and battery life
In its early days, Windows Phone earned a reputation for robust performance that has continued to the present. Until recently, the gap in performance between flagships and entry-level devices had been relatively small because Microsoft imposed restrictions on which processors and other specs could be used on the platform; phones like the Nokia Lumia 520 are cheap, but perform well given their limited processing power and RAM. As Microsoft has slowly lifted those restrictions, flagships have now become even more powerful, possessing the same specs as many top-of-the-line Android devices.
With the One M8 for Windows, you're going to enjoy all of the same performance benefits as the Android version -- on paper, at least. It has a quad-core 2.3GHz Snapdragon 801 chipset with 2GB of RAM, so it's no slouch. The M8 for Windows runs WP 8.1 Update 1, which is still only available as a Developer Preview on other devices; this is the first time it's been included on a new device out of the box.
The M8 for Windows runs well most of the time. To nitpick ('tis my burden and duty), the Android version is a little faster when loading apps and multitasking, primarily due to the time-wasting animations scattered throughout the Windows Phone OS. It's a difference of a couple seconds each time, which likely won't matter at all to most users. Games run smoothly with few to no frame skips, but the same titles on Android were consistently more fluid. Occasionally, the processor on the Windows version would slow down, making games extremely choppy for around 15 seconds, after which it'd smooth out for two or three seconds before returning to its frozen state. A reboot usually cleared up the problem, but I've never run into similar issues on the M8 for Android.
I've also noticed that the two 1080p displays aren't calibrated identically; it appears that both devices use different temperatures (the original M8 is a tad warmer), though the brightness and viewing angles are both good. Images and graphics on the Windows Phone model appear to be fuzzier and have less clarity when looking at the two devices side by side, as if they're displayed at a lower resolution somehow, but at least text and games look great.
Running benchmarks can be a little tricky when doing cross-platform comparisons, since most Android tests aren't available on Windows Phone. Microsoft's OS was slightly better on internet-based tests like SunSpider (609ms vs. 649ms, where a lower time is better) and Google Octane (2,801 vs. 2,666), while the Android version did better on GFXBench 2.7 (28 fps vs. 19 fps). The latter score is the most concerning, as it's a much larger difference than I'd expect to see on two devices with identical hardware. Compared to the Icon, the M8 does better on Octane and GFXBench but worse on SunSpider and WPBench. So, just like it was on the camera, there's no clear-cut answer to which phone is the better choice. Since the differences are minor, only a few power users may be dissuaded by the results.
Battery life is a mixed bag. The benefit in testing identical hardware on competing operating systems is that you can compare the two much more easily and closely study the effect each platform has on power efficiency. In my testing so far, it appears that the WP option is better than the Android version in some ways and worse in others. (My tests are ongoing and I'll continue to add more results as they come.)
First, the good news: You can still get through an entire day of normal usage with a little left to spare when you hit the sack. When using the device for email, taking a few photos, browsing the web and running an occasional app, I was able to get slightly more battery life than on my Android M8. When streaming movies through Netflix, the Windows Phone M8 soundly defeats its Android counterpart. Additionally, it lasted for 12 hours and 10 minutes in our standard video rundown test, an increase of 40 minutes over the Sense-clad version.
I wouldn't recommend playing games without having a charger nearby, however; titles like Asphalt 8 and Frozen Free Fall (my kids' favorite) drain up to 30 battery percentage points per hour, as compared to around 15 percentage points on the Android M8. (As an aside, both versions of the M8 get hot when playing graphics-intensive games for more than 10 minutes, the point where it becomes uncomfortable to hold.) Finally, the M8 for Windows lasted two hours and 35 minutes in the standard WPBench CPU stress test, in which it forces the processor to work at high intensity. That's pretty typical: The Lumia Icon delivered nearly identical runtime of 2:36. (It scored 2:42 on our initial review, which was on older firmware.)
Now that a flagship phone is available on more than one major mobile platform, it's tougher to clearly define its competition; whether you're a fan of Windows Phone, Verizon or nothing more than good phone design, there are plenty of factors to consider. The M8 for Windows is available for $100 on-contract, which is half of what the Android edition cost when it first came out. It may have limited appeal at first because of its exclusivity, but Windows Phone users on Verizon will appreciate it because they can now choose among this and two other flagship devices: the Lumia Icon, which is renowned for its 20-megapixel camera, and the Samsung ATIV SE, which is somewhere between a GS4 and GS5 in terms of specs. The Icon is definitely a well-built phone, but the M8 has more curb appeal thanks to its robust aluminum frame and arched back.
Let's go more into specifics on how it compares with the Lumia Icon, its direct Windows Phone competitor. If you're looking for a superior imaging experience, go with the Icon; the M8 is in no way a horrible picture-taking device, but Nokia's had a lot more time and experience to get things right on Windows Phone than HTC has. (And since the Icon still does quite well in low-light situations, the UltraPixel tech isn't quite as persuasive in this case as it may be on Android.) The screen size is only 0.1 inch smaller on the Icon, but the chassis is significantly shorter and much less slippery. HTC offers a great audio experience with BoomSound; the Icon has a hardware shutter button for faster access to the camera; and you'll also need to decide between capacitive buttons and virtual ones.
In a way, the M8 for Windows is competing against its Android brother as well, but HTC's goal isn't to cannibalize sales. By adding a second platform, it's allowing HTC to reach a new set of smartphone users without the high cost of designing, developing and producing a brand-new device. I also doubt many people have held off on buying the M8 because it wasn't on Windows Phone, as fans of the platform will have already looked at other options already available to them.
This is the first time in years that a user can select a device's hardware and software at the same time; you usually have to choose one and then live with the other. People who already use and love Windows Phone now have a flagship option that isn't the latest Lumia device. This phone won't end the platform wars by showing undeniable proof that Windows Phone trumps Android or vice versa; both devices clearly have pros and cons in different areas, and it ultimately comes down to your personal preference.
If your go-to OS is Windows Phone, the M8 is one of the best options, thanks to great design and solid performance. The Lumia Icon still has a bit of an edge, but you'll be happy with either device. If given the choice between M8s, I'd go with the Android version for now because HTC's had much more time, experience and flexibility to get things right on that platform. The performance is a bit more optimized in most cases, and it comes with more of the HTC-branded functionality that makes the One M8 unique. But isn't it nice to finally have a choice?